This invention relates generally to microwave integrated circuits and more particularly to microwave power amplifiers.
As is known in the art, in sophisticated phased array antennas having a plurality of array elements, the output r.f. power of the array elements is often varied to maintain low side lobes. In many phased array radar systems, high power capabilities are required from the array. High power is provided to these array elements by using a circuit which combines the output power from several unit amplifiers.
It is often desired therefore to vary the output r.f. power from the array elements even when such elements include a plurality of unit amplifiers, combined together. While varying the output r.f. power of the array elements, it is nevertheless desirable to maintain a substantially constant D.C. to r.f. conversion efficiency for the amplifier. Most linear amplifiers such as, for example, a Class A amplifier, consume nearly the same amount of D.C. power regardless of the magnitude of the output signal provided from the amplifier. Therefore, when the output power from such amplifiers is reduced, the D.C. to r.f. conversion efficiency is reduced proportionally.
With conventional techniques using a plurality of amplifiers incorporating a conventional power combiner, the power combining efficiency of the power combiner is drastically reduced if one or more of the devices is turned off to reduce the output power of the overall amplifier. When such an arrangement is used in a large array, for example, a significant amount of D.C. power is wasted. Such a reduction in combining efficiency while trying to vary output r.f. power to maintain low side lobes results in a concominant reduction in the D.C. to r.f. conversion efficiency of the overall amplifier. This arrangement is generally undesirable.